The present invention relates to a feedforward amplifier using two kinds of signal cancel loops for reducing distortion, and more particularly to an improved feedforward amplifier suitable for high-frequency power amplification in radio communication equipment.
In general, a conventional feedforward amplifier is provided with two kinds of signal cancel loops; an error signal extraction loop for extracting a distortion (error) signal by subtracting a main signal component from an amplifier output signal, and an error signal elimination loop for attaining only the main signal component in output by subtracting the thus extracted error signal from the amplifier output signal. In Japanese Patent Laid-open (Kokai) No. Hei 1-198809, there is disclosed a feedforward amplifier arranged for reducing distortion accurately using a pilot signal on a signal cancel loop.
Referring now to FIG. 5, there is shown a general circuit configuration of a feedforward amplifier using a pilot signal. The following explains an error signal extraction loop thereof.
An input signal to a first power divider 11 is branched to a first path 33 and a second path 34. On the first path 33, the input signal is then amplified to a predetermined power level by a main amplifier 13. At this step of operation, distortion occurs due to non-linearity of active elements used in the main amplifier 13, causing intrusion of a distortion signal, i.e., an error signal into an output signal thereof. In addition, a pilot signal generated by a pilot signal oscillator 12 is superimposed on the output signal of the main amplifier 13 through a third power combiner 22. Consequently, a part of the error signal is also caused by the pilot signal. Therefore, an output signal delivered from the third power combiner 22 contains the main signal component corresponding to the input signal fed to the main amplifier 13 and the error signal component. The output signal from the third power combiner 22 is then branched to a third path 35 and a fourth path 36 by a second power divider 14. On the third path 35, the output signal thus branched is applied to one of input terminals of a first power combiner 25.
On the other hand, the input signal branched to the second path 34 by the power divider 11 is applied to the other input terminal of the first power combiner 25 through a first vector adjustor 23. At the first power combiner 25, the two signals applied thereto are added. As shown in FIG. 6, the first vector adjustor 23 comprises a variable attenuator 31 and a variable phase shifter 32.
In the above sequence of operation, a power distribution ratio of the first power divider 11, a power distribution ratio of the second power divider 14, and an attenuation level of the first vector adjustor 23 are set up so that an equal amplitude component will be provided for main signal components of the two signals applied to the first power combiner 25. Further, a phase shift level of the first vector adjustor 23 is set up so that a mutually reversed phase will be provided for the main signal components of the two signals applied to the first power combiner 25. Thus, when conditions of equal amplitude and phase reversal are satisfied, the main signal components are canceled mutually to let the first power combiner 25 output an error signal component only.
At this step of operation, if the conditions of equal amplitude and phase reversal are not satisfied due to variation in temperature or deterioration with age in the main amplifier 13, the error signal component delivered from the first power combiner 25 is mixed with a part of the main signal components, thereby increasing a total power level of output from the first power combiner 25. Hence, to ensure accurate extraction of the error signal component, first level observation means 20 detects an output signal level of an error amplifier 26, and first control means 21 adjusts an attenuation level and phase shift level of the first vector adjustor 23 so that the output signal level of the error amplifier 26 will be minimized.
For automatic operation of the above adjustment, the first level observation means 20 comprises a diode detector and a ripple filter, and the first control means 21 comprises a microprocessor and an AD/DA converter, for example. In such an arrangement, the first level observation means 20 converts a power level of output from the first power combiner 25 into a DC voltage, and according to the DC voltage, the first control means 21 controls the first vector adjustor 23. For this control, such an algorithm as a perturbation method is employed.
Then, the following explains an error signal elimination loop of the feedforward amplifier using a pilot signal.
The error signal component output from the first power combiner 25 is amplified by the error amplifier 26, and it is then applied to one of the input terminals of a second power combiner 16 through a second vector adjustor 19 which is structured similarly to the first vector adjustor 23.
On the other hand, the output signal branched to the fourth path 36 by the second power divider 14 is applied to the other terminal of the second power combiner 16. At the second power combiner 16, the two signals applied thereto are added.
In the above sequence of operation, an amplification factor of the error amplifier 26 and an attenuation level of the second vector adjustor 19 are set up so that an equal amplitude component will be provided for error signal components of the two signals applied to the second power combiner 16. Further, a phase shift level of the second vector adjustor 19 is set up so that a mutually reversed phase will be provided for the error signal components. Thus, when conditions of equal amplitude and phase reversal are satisfied, the error signal components are canceled mutually to let the second power combiner 16 output a distortion-free accurate signal (main signal component).
At this step of operation, if the conditions of equal amplitude and phase reversal are not satisfied due to variation in temperature or deterioration with age in the error amplifier 26, removal of the error signal components becomes insufficient, resulting in undesired error signal components (distortion component and pilot signal component) remaining in the main signal component output from the second power combiner 16. Hence, to ensure accurate removal of the error signal components, second level observation means 17 detects a remnant pilot signal level in output from the second power combiner 16, and second control means 17 adjusts an attenuation level and phase shift level of the second vector adjustor 19 so that the remnant pilot signal level will be minimized.
As shown in FIG. 7, for example, the second level observation means 17 comprises a synchronous demodulation circuit including a mixer 41, a low-pass filter 42 and a DC amplifier 43. In such an arrangement, the second level observation means 17 outputs a DC voltage corresponding to amplitude of the pilot signal, and the second control means 18 controls the second vector adjustor 19 according to the DC voltage. Since the pilot signal causes an error signal component similar to a distortion component from the main amplifier 13, distortion is suppressed sufficiently in output from the second power combiner 16.